Table of Contents
It is the total supply of all forms of moisture emanating (coming) from the clouds and falling to the ground. OR Precipitation is deposition of atmospheric moisture.
- It is the most important phase in the hydrological cycle.
- Precipitation (Ppt) is the immediate source of stream run-off, hence its occurrence, distribution and intensity determine the hydrologic behavior of streams, namely total discharge, discharge regime, and quality of the discharge.
- The mechanism of how ppt is formed in the high atmosphere is a subject matter of ‘meteorology’.
ORIGIN OF PRECIPITATION:
Origin or formation of precipitation may be.
- In the high atmosphere irrespective of soil surface or vegetation cover (rain, snow).
- Near the ground (mist, fog)
- At the ground surface (Dew, rime)
FORMATION OF PRECIPITATION:
Formation of precipitation needs following conditions and processes:
Presence of moisture:
Water vapors’ presence in the atmosphere only conditions for precipitation. Water vapors are always present in cloudy and even in the dry atmosphere.
Condensation nuclei present in sufficient quantity condense to form droplets due to a decrease in atmospheric temperature. These droplets are further condensed to form clouds and in the form of fog near the ground.
Cooling takes place by:
- Mixing of air masses of different temperature by radiation or by the dynamic ascent of air. Such cooling leads only to fog formation.
- The lifting of air produces more cooling which produces:
- Convective precipitation (horizontal and vertical mixing of rising air). It is a light shower to cloudbursts.
- Cyclonic precipitation (Convergence of air currents in low-pressure zones). It is the major source of precipitation in plains, in low lands in monsoon season.
FACTORS AFFECTING PRECIPITATION:
Following are the two main factors affecting precipitation:
At equator highest rainfall; it decreases with increase in latitude
Large evaporation areas:
Coastal areas receive much rain/ ppt.
CLASSES OF PRECIPITATION:
On the basis of Origin:
- Ppt which forms in the high atmosphere and which falls from above irrespective of the characteristics of vegetation and soil surface (from clouds).
- Horizontal ppt near ground surface (fog ppt) which originates in the atmosphere near the ground.
- Ppt such (as dew and rime), which develop at the ground surface itself.
On the basis of States:
- Liquid (rainfall, dew, drizzle, etc)
- Solid (snowfall, hail, storm, etc)
On the basis of Climatologically Observation:
Types of precipitation
- For ppt to occur, three conditions are necessary:
- Water vapor
- Condensation nuclei
- Condensation nuclei are usu available in sufficient quantity to be it as dust particles.
- The capacity of the atmosphere to retain moisture is largely determined by its temperature.
- When the temp cools below the dew point, water vapor begins to condense and to form droplets which gradually grow.
- This process becomes visible in the form of clouds, or near the ground as fog.
- Eventually, when the droplets combine to form bigger drops, water will begin to fall out.
- The large-scale cooling needed for significant amounts ppt is achieved by the lifting of air.
- Precipitation is often typed according to the factor responsible for lifting and cooling of the moist air.
- Energy form the source ‘sun’ reaches the earth by passing through different zones in the form of rays.
- On reaching the atmosphere, these reduce the bulk of air and increases its temperature
- With less bulk, light air tends to rise in a cooler, denser, surrounding.
- For every 200 ft, 1OC temp is reduced.
- By vertical convection, the ascending air expands and in consequence cooling dynamically
- This leads to convective precipitation.
- The difference in temp b/w air masses may be caused by unequal storms (or, it can be caused by differential cooling of the upper air layers.)
- Convective ppt is spotty, and its intensity may range from light showers to cloudbursts.
- This is the main type of ppt occurring in the plains and in fact, much of the monsoon ppt in the lowlands of Pakistan is of this type. (80 to 90% )
- As solids heat and cool more quickly than liquids:
- A land mass is, therefore, heated more quickly in summer and cooled more easily in winter than the oceans which surround it and so does the air above the land mass.
- In summer over the hot land mass, the air heats and rises and a low-pressure zone or cyclone is created.
- High pressure, anticyclones, persists over the neighboring cooler oceans, and air currents begin to converge from the high-pressure zones to low-pressure cyclones.
- As these currents meet, they ascend.
- The summer monsoon in Pakistan originates from this unequal heating b/w the oceans (the Indian Ocean and the Arabian Sea)
- In winters, the mechanism is reversed, and winters blow off-shores (NW monsoon).
In conjugation with the convergence of air masses of different temp in low-pressure cyclones, fronts (a line along which one mass of air meets another that is different in temperature or density) may be formed and air can be forced to ascend mechanically in that warmer, lighter air glides over denser, cooler air.
- Occasionally, also, cold air gets wedged under warm air
- In this way, frontal storms, originate out of the conflict of warm and cold air masses.
Frontal storms are caused by the force of a layer of warm air over a layer of colder as a longer period. In cold front, warm moist air produces intense rains over a small area. In a warm front, cold air produces rain falls over a large area.
- Results from the mechanical lifting of warm moist air over mountain barriers.
- It is causal by lifting of air when traversing mountain range laying across their paths.
- Much of the rain in the outer Himalaya hills both during the summer monsoon and in winter.
The distinction b/w convective, cyclonic, frontal, and orographic ppt is to certain extent academic as well as all types can occur in nature in combination, and no sharp boundaries exist b/w them.
On the basis of Geographical Effects (Geographic variations):
- The amount of ppt is large in coastal zones where evaporation from the oceans supplies much water vapor into the atmosphere.
- Consequently, the amount of ppt often diminishes with increasing distance from the coast.
- Since, lifting and cooling of air is an important condition. As soon as airlifts, its temp decreases. When it falls down the freezing point and particles get a dia of 1mm rain forms.
- Similar to Orographic ppt
Windward and leeward direction areas:
- Since lifting and cooling of air is an important condition, we find high ppt and frequent rainfall on the windward sides of mountain ranges.
- By contrast, on the leeward direction, ppt quickly diminishes, and in fact, arid conditions may obtain.
FORMS OF PRECIPITATION
- Ppt can occur in one of the several forms
- It may change from one form into another
- Basically, we distinguish b/w liquid (rain) and solid precipitation (snow)
- Different forms of precipitation can be divided into
- A. Liquid form
- B. Solid form
|Form||Average drop size (dia)||Fall velocity|
|Cloud burst||5.6-6.5mm||Up to 8.0m/s|
|Fog ppt & Mist||–||–|
Liquid form of precipitation includes:
- Dew: water droplets which are deposited by direct condensation from the surrounding are layers on horizontal surfaces.
- Fog: cloud-like mass or aggregate or layer of minute globules of water ice crystals suspended in the atmosphere or near the earth surface.
- Mist: thin fog in which horizontal visibility is greater than 1 mm where water vapors are smaller than fog
Solid form of ppt includes:
- Snow: – ppt in the form of ice crystals results from sublimation i.e. from water vapors directly to ice.
- Snow flank: – is made up of a number of ice crystals fused together.
- Hail; – is ppt in form of balls or bumps of ice crystals over 5mm diameter formed by alternate freezing and melting as they are carried up and down in highly turbulent air currents.
- Sleet: – is frozen raindrops cooled to the ice state, while falling through the air at subfreezing temperatures.
- Glaze: – is the ice formed when drizzle or rain freezes as it comes in contact with the cold object at the earth.
- Rime & Frost – is light feathery sidle like ice crystals which are deposited to when the dew point temperature is below freezing point.
- Grapple: – It is a form of snow in soft pellets having Non –uniform surface.
MEASUREMENT OF RAIN:
Hydrologists use an instrument called an automatic recording rain gauge to measure precipitation. The instrument weighs and charts rainwater as it falls, keeping a record on a paper strip. The record shows the time, duration, and intensity of rainstorms. Hydrologists may also use nonrecording gauges, clear tubes with markings in millimeters or inches on them. Light, intermittent snowfall is measured by the same instruments as rainfall. In areas where snow often accumulates to great depths, gauges in the form of collecting tanks store the snow, sometimes over a whole season. To measure the depth of liquid water to which the snowfall is equal, the snow is mixed with a salt brine to melt it as it enters the gauge. Snowfall is also measured with a snow tube, which is inserted into an accumulation of fallen snow to get a sample that is weighed to determine the depth of water equivalent.
Precipitation is any form of water (either liquid or solid) that falls from the atmosphere and reaches the ground, such as rain, snow, or hail. Rain gauges are instruments that measure rainfall. The standard rain gauge consists of a funnel-shaped collector that is attached to a long measuring tube.
MEASUREMENT OF HUMIDITY:
Humidity refers to the air’s water vapor content. Hygrometers are instruments that measure humidity. The maximum amount of water vapor that the air can hold depends on the air temperature; the warm air is capable of holding more water vapor than cold air. Relative humidity is the ratio of the amount of water vapor in the air compared to the maximum amount of water vapor that the air could hold at that particular temperature. When the air is holding all of the moisture possible at a particular temperature, the air is said to be saturated. Relative humidity and dew-point temperature (the temperature to which air would have to be cooled for saturation to occur) are often obtained with a device called a psychrometer. The most common type of psychrometer is a sling psychrometer. This instrument consists of two thermometers mounted side by side and attached to a handle that allows the thermometers to be whirled. A cloth wick covers one thermometer bulb. The wick-covered thermometer bulb (called the wet bulb) is dipped in water, while the other thermometer bulb (the dry bulb) is kept dry. Whirling both thermometers allows water to evaporate from the wick, which cools the wet bulb. By looking up the dry and wet bulb temperatures in a set of tables, known as humidity tables, it is possible to find the corresponding relative humidity and dew-point temperature.
EVALUATION OF PRECIPITATION RECORDS/ FIND NUMERICAL VALUE EXPRESSION OF PRECIPITATION:
Amount of rainfall measured in depth to which rain water accumulated observed site.
The amount of ppt may be calculated in two dimensions:
- Mean rainfall over an area
- Total vol of water received over catchment evaluation needs
- Collection of individual station rainfalls
- Collection of rainfall of area, sub-areas
- Inference of data for an area, sub-areas
Method of Inference/ Evaluation:
Following are the methods to evaluate:
Catchments having more than two rain gauges take the arithmetic mean and multiply it with the area gives vol of rainfall.
This method was developed by Thiesen:
- Catchment map along with the location of rain gauges
- Connecting location points on the map by straight lines (connecting line).
- Perpendicular bisecting lines on connecting lines and originating from mid-point of connecting lines. These lines form polygons called Theisen polygons. Drawing of lines is based on trial and error method to a minimum length of lines.
Hav = (H1F1 + H2F2 + ………… + HnFn) / F
(Where; Hav = average rainfall; H1, H2 = depth of rainfall in different rain gauges; F1, F2 = area of polygon; F = total catchment area)
Limitations of Polygon Method:
- Change of location of rain gauge during observation makes it tedious.
- Variation of rainfall due to topography is not taken into consideration (orographic factor of topography)
More accurate as an orographic factor considered stops:
- Mean depth of rainfall measured at different stations for a desired period (year, month, day, individual storm)
- Plotting means depth of rainfall on a map at the geographic location of stations.
- Lines of equal/ similar precipitation are drawn called Isohyets. Isohyets should follow contour lines as much as possible.
Considerable experience and observations are required. The mean precipitation may be calculated as follows:
Hav = (H1F1 + H2F2 + ………… + HnFn) / F
(Where; Hav = average rainfall; H1, H2 = rainfall values; F1, F2 = sub-areas of appurtenant; F = total catchment area)
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